1. Field of the Invention
The present invention relates to an image forming apparatus configured to form an image on a recording medium by using an electro-photographic system such as a copier, a printer, a facsimile machine, and a multi-function printer.
2. Description of the Related Art
Conventionally, there is known a tandem-type image forming apparatus configured to form an image by using an endless intermediate transfer belt among image forming apparatuses such as a copier, a printer and others. This image forming apparatus is configured to form toner images of yellow, magenta, cyan and black by a plurality of image forming portions each including a photosensitive drum, to primarily transfer the toner images by superimposing on the intermediate transfer belt, to secondarily transfer the toner images to a recording medium (sheet), and to fix the toner images on the recording medium by pressure and heat.
In general, the intermediate transfer belt is stretched around a driving roller configured to apply a driving force to the intermediate transfer belt, a tension roller tensioning the intermediate transfer belt with a certain tension to form a transfer surface on which the toner images are transferred from the image forming portions, and a stretch roller such as a secondary transfer inner roller. A transfer bias is applied to this secondary transfer inner roller such that the toner images on the intermediate transfer belt are secondarily transferred to the recording medium.
By the way, if a delivering direction of the stretch roller is inconsistent with a direction of the intermediate transfer belt delivered by the driving roller when the intermediate transfer belt is rotationally driven, so-called meandering by which the intermediate transfer belt moves gradually in a direction (main scan direction) orthogonal to a rotating direction of the intermediate transfer belt occurs. If such meandering of the intermediate transfer belt occurs, the toner images formed by the plurality of image forming portions shift in the main scan direction, causing color shift of the respective colors.
In order to prevent such meandering of the intermediate transfer belt, Japanese Patent Application Laid-open No. 2000-34031, for example, discloses a meandering control system configured to correct meandering of the intermediate transfer belt by tilting a steering roller supporting the intermediate transfer belt. Japanese Patent Application Laid-open No. 2013-3381, for example, also discloses a meandering control system configured to move an inclined position of the steering roller in response to attachment/detachment of rotors disposed around in contact with the intermediate transfer belt to overcome a problem that the inclined position of the steering roller where travel of the intermediate transfer belt is stabilized is changed by the attachment/detachment of the rotors.
However, a time until when the travel of the intermediate transfer belt is stabilized by the meandering control and the inclined position of the steering roller vary depending on fluctuations of a coefficient of friction between an inner surface of the intermediate transfer belt and a surface of the steering roller. This occurs because traveling speed of the intermediate transfer belt with respect to an inclination of the steering roller varies, and the time until when the travel of the intermediate transfer belt is stabilized also varies. For instance, if the time until when the travel of the intermediate transfer belt is stabilized is prolonged, a state in which the travel of the intermediate transfer belt is unstable occurs before starting to form an image. Color shifts occur in this state because the intermediate transfer belt travels aslant in the main scan direction, making it difficult to output a high quality image.
The move of the inclined position of the steering roller where the travel of the intermediate transfer belt is stabilized occurs also by the application of the transferring high-voltages (primary and secondary transfer biases) for transferring the toner images to the intermediate transfer belt. This problem occurs because a direction of a rotational axis of the photosensitive drum of each color disposed in contact with the intermediate transfer belt is not at right angles to a traveling direction (rotating direction) of the intermediate transfer belt and becomes obvious by an action on absorption power generated by the application of the transferring high-voltage.
In view of the state described above, it is necessary to move the inclined position of the steering roller corresponding to a status of the image forming apparatus in order to quickly stabilize the travel of the intermediate transfer belt. To that end, there arises a problem that the inclined position corresponding to the operation condition of the image forming apparatus must be detected and to update the inclined position at any times.